Through this change in legal jargon, Instagram appears to be transforming itself from an image-sharing service into an image-taking service, but the consequences of the altered terms remain unclear. Writing for The Verge, Nilay Patel insists the new language represents an improvement in some ways because "the old Instagram terms allowed for modification, but the new ones don't."

Instagram "does not claim ownership of any Content that you post," yet it claims rights almost equivalent to ownership, in the form of "a non-exclusive, fully paid and royalty-free, transferable, sub-licensable, worldwide license to use the Content that you post."

The terms that have users alarmed concern Instagram's right to sell to advertisers users' images for inclusion in their marketing: "To help us deliver interesting paid or sponsored content or promotions, you agree that a business or other entity may pay us to display your username, likeness, photos (along with any associated metadata), and/or actions you take, in connection with paid or sponsored content or promotions, without any compensation to you."

The revised terms of service go into effect on Jan. 16, 2013.

Instagram's terms of service now conform with that Facebook already does with user content. And they don't appear to be that different from the terms imposed on anyone posting content to an ad-supported online service.

The Google+ terms of service, for example, state, "When you upload or otherwise submit content to our Services, you give Google (and those we work with) a worldwide license to use, host, store, reproduce, modify, create derivative works (such as those resulting from translations, adaptations or other changes we make so that your content works better with our Services), communicate, publish, publicly perform, publicly display and distribute such content."

Google, however, insists there's a difference. "As our Terms of Service make clear, 'what belongs to you stays yours,'" a Google spokesperson said in an email. "You own your files and control their sharing, plain and simple. Some of our Services allow you to submit content. You retain ownership of any intellectual property rights that you hold in that content. In addition, on Google+ you can export your photos and other data whenever you'd like."

Yet Instagram also claims that users own their files. So whatever it is that users of online services own, they own less of it once files are shared online.

Santa Clara University law professor Eric Goldman said in a phone interview that it's not obvious exactly what rights Instagram is claiming because its privacy policy comes into play, too. He said a lot will depend on how Facebook interprets its legal language and what it's trying to accomplish.

"It seems like Facebook is floating trial balloons about language in its user agreement to stuff a lot of ads in our face," he said.

Facebook did not immediately respond to a request for comment.

In any event, Instagram's terms of service change has outraged users. On Facebook's Instagram page, Instagram user Nicolas Emery commented, "Sorry to read Instagram's updated terms including the right to sell my photos with no compensation or credit. I enjoyed using Instagram but have no hesitation in deleting your app under these terms."

Many users echoed that sentiment. Others proposed using Instagram henceforth only to capture inappropriate or offensive images, under the assumption such snapshots would be worthless to advertisers.

In a blog post, Kurt Opsahl, an attorney with the Electronic Frontier Foundation, urged Instagram to reconsider its policy "because it conflicts with the three key principles we developed for social networking services: informed decision making, control and the right to leave."

Users, he said, cannot make an informed decision about how their images might be commercialized, cannot opt out and cannot remove their images from Instagram after the new terms of service take effect. Opsahl suggests that Instagram consider image licensing and revenue sharing more along the lines of what Flickr has done.

Flickr, a pioneering online image sharing service that many feared would dwindle into irrelevance until Marissa Mayer took the helm at Yahoo, might find new users among those disillusioned with Instagram. Flickr addressed the issue of photo ownership last year, noting, "We feel very strongly that sharing online shouldn't mean giving up rights to your photos. Our Terms of Service clearly spell out that Flickr/Yahoo doesn't own the photos that you upload. You, as a member, maintain all ownership rights to the photos that you upload to Flickr."

Whatever that means.

Story update:

Instagram co-founder Kevin Systrom has acknowledged in a blog post that users are upset and attempts to clarify some of the confusion about Instagram's new terms of service.

"Our intention in updating the terms was to communicate that we'd like to experiment with innovative advertising that feels appropriate on Instagram," he wrote. "Instead it was interpreted by many that we were going to sell your photos to others without any compensation. This is not true and it is our mistake that this language is confusing. To be clear: it is not our intention to sell your photos. We are working on updated language in the terms to make sure this is clear."

Systrom says Instagram plans "to modify specific parts of the terms to make it more clear what will happen with your photos."